The present embodiments relate to an output stage module for a power amplifier device, such as a power amplifier for a magnetic resonance device.
Magnetic resonance devices are used primarily for clinical applications. As a part of the imaging process, nuclear spins of an object under examination aligned via a transmit antenna are excited, and image data is received by a receive antenna. High power is needed to operate the transmit antenna. The high power is provided by a power amplifier device. The power amplifier device may be built into a transmit unit of the magnetic resonance device.
In such cases, a power amplifier device, in which as many components and/or conductor structures as possible are implemented on a common printed circuit board structure, is produced. The power amplifier device has a number of power modules (e.g., four power modules) that may generate an output power of 5-8 kW, so that, in combination, a power of, for example, 30 kW may be achieved. The power modules, which ultimately represent an output stage, may also be considered as output stage modules. A push-pull output that operates in accordance with the push-pull principle with symmetrical input signals may be used.
Since the system operates at extremely high power, transistors used in the output stage modules need to be cooled. In known power amplifier devices, this is realized by a copper plate being arranged below the printed circuit board. Coolant channels running to positions to be cooled are routed through the copper plate. The printed circuit board is furnished with breakthroughs at locations provided for the transistors, so that the housed transistor units provided are inserted at the breakthroughs. The housed transistor units may be brought into direct contact with the copper plate provided below the printed circuit board.
The transistor units having their own housing used for this purpose have a plurality of disadvantages. The transistor units are attached by reflow soldering (re-melt soldering) and may only withstand three reflow cycles. The first assembly occupies two of the cycles so that only one reflow cycle remains for repair. The replacement of the damaged transistor may adversely affect the solder quality of the other installed transistors. This technical problem stems from the overall construction of the printed circuit board of the power amplifier device. The desired compact structure demands a flangeless transistor unit that is to be soldered to the copper plate, which is connected to the printed circuit board. In addition, this solution is expensive not only in relation to the manufacturing of the transistor unit with the housing, but also in relation to the printed circuit board arranged on the copper plate. It is extremely complicated to replace a damaged transistor. A loss of power is risked during the repair processes.